There is known an automatic transmission of a type incorporating a plurality of frictional coupling devices including a plurality of clutches which have respective clutch drums for operating the clutches. For reducing the required axial dimension of the automatic transmission, one of the clutch drums is disposed within another of the clutch drums. JP-5-33833 A discloses an example of such type of automatic transmission which incorporates two clutches operable to connect one rotary member to respective two other rotary members, and in which the clutch drum of one of those two clutches is fitted in the clutch drum of the other clutch, whereby the required axial dimension of the automatic transmission can be reduced.
In the automatic transmission disclosed in the above-identification publication, two fluid passages through which a working fluid is supplied to advance pistons of the two clutches to force friction plates against each other are formed in the axial direction of the automatic transmission, at substantially the same radial position. Although the required axial dimension of the automatic transmission can be reduced owing to the arrangement in which one of the two clutch drums is disposed radially inwardly within the other clutch drum, the arrangement of the two fluid passages described above undesirably results in an increase of the required axial dimension of the automatic transmission, so that the automatic transmission has a comparatively large axial overall length.